treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/osbase/net/dns/forward/proxy/connect.go b/osbase/net/dns/forward/proxy/connect.go
index d1c5b8e..a11e8e6 100644
--- a/osbase/net/dns/forward/proxy/connect.go
+++ b/osbase/net/dns/forward/proxy/connect.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package proxy implements a forwarding proxy. It caches an upstream net.Conn
 // for some time, so if the same client returns the upstream's Conn will be
 // precached. Depending on how you benchmark this looks to be 50% faster than